1. Field of the Invention
This invention relates to electrolysis of molten alkali hydroxide to produce alkali metal and more particularly it relates to apparatus for improving energy efficiency by anolyte dehydration.
Electrolysis of molten sodium chloride has replaced electrolysis of molten sodium hydroxide as a principal source of sodium metal. Sodium chloride is the principal sodium mineral and a high current efficiency results from insolubility of gaseous chlorine in the anolyte. Yet energy efficient electrolysis of molten sodium hydroxide as an alternative source of sodium metal has the following advantages:
sodium hydroxide is co-produced with hydrochloric acid in large commercial quantities by electrolysis of aqueous sodium chloride solutions but present demand for the sodium hydroxide tends to be less than for the hydrochloric acid; PA1 the decomposition potential of sodium hydroxide is substantially lower for a thermodynamic advantage; and PA1 sodium hydroxide is less corrosive and has a lower melting temperature for processing advantages.
Another field of application for the invention is reduction of a sodium-potassium hydroxide mixture in a cyclic secondary energy system wherein the eutectic alloy of sodium and potassium called NaK is a liquid fuel for automobiles as disclosed in U.S. Pat. Nos. 3,911,284 and 3,911,288 issued to the applicant. In this energy system, NaK metal reacts with water and atmospheric oxygen is automobile engines to provide mechanical power. NaK hydroxide, which is formed as a reaction product, is exchanged for Nak metal during a fuel stop. Electrolytic reduction, recycled fuel material, and nuclear or renewable primary energy sources provide an automotive system which is environmentally benign and, particularly, does not contribute to atmospheric carbon dioxide with its potentially adverse effects upon climate.
2. Prior Art
A commercial electrolysis cell for producing alkali metal from molten alkali hydroxide was disclosed by H. Y. Castner in U.S. Pat. No. 452,030 (1891). Castner's cell comprises an iron or nickel anode and cathode immersed in molten electrolyte and separated into an anolyte portion and a catholyte portion by a diaphragm such as a nickel screen or wire gauze. The basic reaction is: EQU 2NaOH.fwdarw.2Na(cathode)+H.sub.2 O+1/2O.sub.2 (anode).
The sodium metal dissolves in the catholyte, the water dissolves in the anolyte, and the oxygen is substantially insoluble. The dissolved water is electrolyzed so that the overall electrolysis of the 2NaOH is: EQU 2Na+H.sub.2 (cathode), and EQU O.sub.2 (anode).
Since half of the current is expended to electrolyze formed water, the basic Castner cell is limited to a current efficiency of 50%. Some of the sodium may diffuse to the anode and react with the forming water to form NaOH and hydrogen which reduces sodium metal yield and further decreases current efficiency. Castner's improvements, which included the diaphragm to prevent convective interactions of water in the anolyte with sodium in the catholyte and further included operation near the electrolyte melting temperature to reduce diffusion of sodium metal, made early commercial use feasible but did not overcome inefficiencies resulting from formed water dissolved in the anolyte.
Although stable hydrates of sodium hydroxide do not exist above 145.degree. C., direct thermal dehydration is difficult. At the 320.degree. C. melting point of sodium hydroxide, water content is reduced to only about 10%. With a temperature of 360.degree. C. at a reduced pressure of 500 mm. Hg, a 1% water content is expected. Commercially anhydrous levels are attained at 500.degree. C. in open pots. Sodium hydroxide can be dehydrated by azeotropic distillation with kerosene at 220.degree. C. But the various thermal processes for dehydrating sodium hydroxide are either not sufficiently effective or incompatible with operating conditions within Castner cells.
One method for use within Castner cells for reducing anolyte water content is based on additives which reduce water solubility or which react with the water. Sodium iodide is an example of the first and sodium amide of the second. Sodium amide and water form ammonia and sodium hydroxide. Additives, however, tend to increase process complexity and have not been successful.
Another method for reducing anolyte water content is described by F. J. Dobrovolny in "Official Gazette" 1950, Vol. 637, pages 1575-6. The anolyte is circulated through a heating zone where it is flushed with an inert gas to remove water vapor. Current efficiency of the Castner cell is improved but such thermal dehydration is not energy efficient at the high flow rate of sodium hydroxide needed to maintain low levels of water in the anolyte.